British intelligence agents are unlikely to have been involved in the abuse of terror suspects without the approval of ministers, the former head of MI6 said.

In an interview at the Guardian Hay festival, Sir Richard Dearlove said that no British agent would be involved in "questionable practices" without first getting legal advice and political support.

He was asked by the human rights lawyer Philippe Sands QC whether growing evidence pointing to British collusion in torture meant that there had been ministerial sign-off from Tony Blair's government.

Dearlove said: "That's a speculative question, [but] there should have been."

He said that the intelligence community was "sometimes asked to act in difficult circumstances. When it does, it asks for legal opinion and ministerial approval … It's about political cover."

His comments follow allegations of British involvement in the torture of British citizens in Egypt and Bangladesh, and growing evidence pointing to MI5's complicity in the torture of British citizens and others in Pakistan.

Two high court judges say they have seen "powerful evidence" of the torture of one man before he was questioned by an MI5 officer, and a court heard that MI5 and Manchester police drew up questions for a Pakistani intelligence agency to put to a man who subsequently had three fingernails pulled out.

The high court in London heard evidence of the existence of an official British interrogation policy, during cross-examination of an MI5 officer known as witness B. The officer had questioned former Guantánamo Bay detainee Binyam Mohamed, a British resident who says MI5 officers colluded in his torture.

Witness B said: "I was aware the general question of interviewing detainees had been discussed at length by security service management legal advisers and government, and I acted in this case, as in others, under the strong impression that it was considered to be proper and lawful."

Dearlove, who was head of the Secret Intelligence Services (MI6) from 1990 to 2004, told the Hay festival he was not personally aware of any British complicity in torture. "The UK position is crystal clear," he said. "We don't use torture and we are actively opposed to it. In instances where we know that governments are not acting in line with our legislation, we express our disagreement and disapproval."

He said Britain had never adopted the US concept of a "global war on terror" which denied detainees rights under international law. "I had concerns about the US's harsh treatment of detainees. We had different views of the legality of these practices," he said.

One of the main concerns was the rendition of terror suspects . "On rendition we absolutely took a different view," he said. "We were aware that rendition was going on but not the details. We were aware of some cases."He also revealed that Britain and the US had met in June 2001, before the September 11 attacks, to discuss fears of a major terror attack. "It was a routine meeting that turned into something that wasn't routine," he said.

An increase in "chatter" picked up on surveillance networks and an accumulation of intelligence suggested that a major plot was afoot – although neither American not British officials suspected that it would take place on US soil.

But he said that he felt the US's response to 9/11 had been disproportionate.

"I'm a great believer in proportionality," he said. "And while what happened on 9/11 was a dreadful and serious event, in no way did it threaten the integrity of western civilisation. It was not a nuclear attack on New York, let's be clear on this."